1. Technical Field
The invention relates to optical waveguides and photonic circuits. In particular, the invention relates to optical waveguide ring resonators used in photonic systems.
2. Description of Related Art
An optical ring resonator generally comprises a segment or segments of optical waveguide arranged as a ring-shaped, closed loop. Such optical ring resonators have a variety of important uses in photonic systems. For example, the optical ring resonator may be part of a photonic filter. In another example, the optical ring resonator may be employed as an element in a photonic frequency multiplexer used to select a particular frequency portion of an input optical signal. The frequency multiplexer may further route the selected frequency portion along an optical path that differs from that of the input optical signal. If the optical ring resonator is rendered controllable, the photonic frequency multiplexer may be employed as an optical switch, for example. In another example, one or more controllable optical ring resonators also may be used adjust a performance of the photonic filter.
An optical ring resonator may be rendered controllable by adding a control electrode to an optical path within the resonator. The control electrode, when activated, acts to change a refractive index of a material of the optical waveguide in a vicinity of the control electrode. The change in refractive index interferes with or disrupts a resonance of the optical ring resonator. In particular, the activation of the control electrode may prevent an optical signal from being coupled into and resonating within the optical ring resonator. Thus, the control electrode essentially allows the optical ring resonator to be turned ‘ON’ and turned ‘OFF’ according to an input that controls the control electrode.
To be effective for such ON/OFF control, the control electrode must generally be relatively large. In particular, the control electrode typically encompasses a significant portion or length of the optical path within the optical ring resonator. Unfortunately, such relatively large control electrodes exhibit a relatively large capacitance. The relatively large capacitance of such control electrodes can and does adversely affect a performance of the controllable optical ring resonator. For example, the relatively large capacitance may limit an upper frequency of the controllability of the optical ring resonator. In addition, large control electrodes may interfere with an optical field of the optical signal resulting in an increased optical loss in the optical path. The increased optical loss may be particularly pronounced if the electrodes comprise a metal. Moreover, the large control electrode provides little or no control flexibility.